Inorg. Chem. 1984, 23, 4412-4417
4412
Recent work4*has shown that sulfide-Hr is stabilized in the mixed-valence Fe(II1) ( S = 5 / 2 ) + Fe(I1) (S = 2) oxidation ground state. level, which is spin coupled to give a S = Valence states are trapped on the Mossbauer time scale. Consequently, the Fe2S portions of sulfide-Hr and [Fe(salen)12S are not isoelectronic. However, in DMF solution [ Fe(salen)],S undergoes a well-defined electrochemical reduction at EI12= -1.27 V vs. SCE (Up = 80 mV, i,,/i,, = 1). The corresponding reduction of [Fe(salen)],O in DMF occurs at -1.07 V. In Me2S0 solution the potential is -1.05 V.49 [Fe(salen)12S also exhibits a one-electron oxidation in DMF at -0.28 mV (Up = 90 mV, iP,Jipaz 1). [Fe(salen)],O is reported to afford only poorly resolved electrochemical oxidation, but its diiron(II1,IV) level has been reached by chemical oxidation and i~olated.~ These observations raise the possibility of isolating or generating the species [ Fe(sa1en)l2S+>-,with the anion being isoelectronic with the currently characterized state of sulfide-Hr. For the latter, available evidence does not permit a distinction between the proposed47 structures Fe-S2--Fe and Fe...Fe-S2- . Such a distinction may be possible upon elucidation of the properties of [Fe(salen)],S-. In summary, this research provides characterization of the only structurally authenticated example of the Fe(II1)-SFe(II1) single bridge. The set [Fe(~alen)],S-~~,+ (stability permitting) provides an attractive opportunity for further study of structural and electronic properties mediated by a sulfide bridge, including trapped vs. delocalized valence states and magnetic coupling.
3(
2f
2c
’
T
15
5
IC
5
0 I
(nm)
Figure 4. Absorption spectra of [Fe(salen)12Sand [Fe(salen)],O in DMF solutions in the UV-visible region. [A-, nm ( 4 1 : 490 (6930), 348 (23500); 360 (19200), 355 (20400).
Possible Biological Relevance. Aquomet-Hr and H2S or sulfide at pH -8 form a species, sulfide-Hr, with the composition 2 Fe:S2-.47,48Sulfide-Hr is a chromophore with a band at 510 nm (eM 1200), suggesting, as previouisly recognized,47 a relationship with [Fe(salen)],S and [Fe(HEDTA)12S2-.
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Acknowledgment. This research was supported by NIH Grant GM 28856. NMR and X-ray diffraction equipment were obtained by NSF Grants CHE 80-00670 and CHE 800889 1. Supplementary Material Available: Listings of anisotropic temperature factors, calculated hydrogen atom positions, magnetic susceptibility data (22-291 K), and observed and calculated structure factors for [Fe(salen)12S(21 pages). Ordering information is given on any current masthead page. (47) Freier, S. M.; Duff, L. L.; Van Duyne, R. P.; Klotz, I. M. Biochemistry 1979, 18, 5372. (48) Kurtz, D. M., Jr.; Sage, J. T.; Hendrich, M.; Debrunner, P. G.; Lukat, G. S. J . Biol. Chem. 1983, 258, 2115. (49) Wenk, S. E.; Schultz, F. A. J. Eleciroanal. Chem. 1979, 101, 89.
Contribution from the Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02 139
Linear Polysiloxanes from Dichlorosilane DIETMAR SEYFERTH* and CHRISTIAN C. PRUD’HOMME’ Received June 19, 1984
Polysiloxanes of type RR’R”SiO(SiH20),SiRR’R” (R, R’, R” = Me, Me, Me; Me, H, H; Et, Et, Et; Me, Me, H) have been prepared by three different methods: (1) reactions of C1SiH2O(SiH2O)$iH2C1with CH,MgBr, Me3SiOH,and Et,SiOH; (2) H2S04-catalyzedequilibration of cyclic [H2SiOInoligomers with Me,SiOSiMe,; (3) cohydrolysisof H$iCl2 with Me3SiCl and Me2HSiCIusing NaH2P04/Na2HP04-bufferedmedia. Lower species, Me2RSiO(SiH20),SiMe2R (n = 1-3), were isolated and characterized (R = Me, H). Introduction In an earlier paper2 we described the cyclic polysiloxanes, [H2SiO],, which are obtained when dichlorosilane, H2SiC12, is hydrolyzed under the appropriate condition^.^ Also of ( 1 ) Postdoctoral Associate, on leave from Rh6ne-Poulenc Recherches, Lyon. (2) Seyferth, D.; Prud’homme, C. C.; Wiseman, G. H. Inorg. Chem. 1983, 22. 2163.
0020-1669/84/1323-4412$01.50/0
interest to us were linear siloxanes of the type RR’R”Si0[S~HZOI~S~RR’R’’, where R, R’, and R” can be the Same or different. We report here our studies directed toward this goal. Linear Polysiloxanes containing H2SiO units in the Polysiloxane chain have been reported in the patent literature. (3) Previous studies of the hydrolysis of-dichlorosilane are discussed in ref 2.
0 1984 American Chemical Society
Linear Polysiloxanes from Dichlorosilane Table I. Cohydrolysis of Dichlorosilane and Trimethylchlorosilane amtof amtof amt of siloxane siloxanes, Me,SiCl, H,SiCI, , Me,SiCI/ mmol mmol H,SiCI, yield,% mmol (wt %) 664 166 4 0 189 (55.5) 90 1 78 (29.2) 2 20 (9.4) 3 2.9 (1.6) 4 (0.1 (
